Simulation of Crystal Nuclei at the Liquid-Air Interface toward Morphology Control via Surface Tension

Surface tension is a crucial factor in the crystallization of organic semiconductors from solutions. However, the current conclusions based on experimental results have been primarily empirical and qualitative, making it hard to distinguish the role of surface tension from other solvent properties. In this study, we employed both molecular dynamics simulations and experiments to investigate crystal nuclei behavior at the liquid-air interface. Through equilibrium simulations and free energy calculations, we quantitatively described the moving behavior of nuclei in various solvents, relating it to an interfacial transition, in terms of interfacial tension. Nuclei tend to remain at the liquid-air interface if the surface tension of solvents is higher than the surface free energy of nuclei but sink into the bulk liquid otherwise. Furthermore, aside from governing the nuclei behavior, surface tension also determines the crystallization location, as confirmed through growing crystals on substrates with Ag nanowires as impediments. By clarifying the role of surface tension, our research contributes to the knowledge of solution crystallization of organic semiconductors and the establishment of a framework for tuning processing conditions and crystal morphology for future studies.